For our first ever Summoner's Guidebook off of Massively, we'll talk about blaming your team. Let's be serious for a moment here. If your team loses, it's probably your fault. I don't care how good your KDA is. I have seriously seen commenters post about how they went 20+ kills with single digit deaths and still lost. I will be completely honest; if you had a score that good and somehow lost, you were to blame.
Bronzies can't be bothered to play well, sure. I understand that there are bad players. I understand that there can be lanes that feed hard and make it difficult to win. However, if you are somehow able to have a good score in spite of that, it is very likely that you can win. The enemy team is failing to shut you down, and you are doing very well when your other team isn't. This means that if you can lead your team, you can win.
Of course, that is impossible if you were a giant flaming jerk the whole game. Stop it and start leading. From when things go downhill, you start positively motivating your team. Encourage them to play more cautiously and that things can be turned around. Then, place wards and make plays off of them. Then encourage your team to place wards. Ping where you want them, and more importantly, when you want them. Don't tell a teammate to ward somewhere dangerous when the enemy positions are unknown. Eventually, between kills and objectives you can distribute some gold to the rest of your team.
If you're being a jerk, you cannot do that. Don't flame your team and try to 1v5, that doesn't work and you're destined to lose. If you are the team's star player, you are your team's only hope, so push your team up and help them.
Really this article is not about that, though. I can talk more about leveraging advantage somewhere else. This is just about mindset. Top players don't generally blame their teams. Yes, it sucks when a lane feeds or your jungle is useless. However, they are always thinking about "if I can play better/do X, I can turn the game around." They are soft on their teams and hard on themselves.
If you actually want to get better at League of Legends, you need to be focused on what you can do or what you can get your team to do. This is more of a life lesson than anything. You can't fix other people, but you can fix yourself. I should say that you can fix other people, but if you really wanted to you would blog and make tutorials and strategy videos and other such things.
You have to focus on what you can do and not dwell on what you can't. Some games are lost no matter what, but those aren't the games where you get 23 kills. In general, the games that you can't fix are the ones where the enemy mid laner/jungler/support gets fed and roams everywhere and shuts everything down. If the game is lost, generally the enemy team doesn't let anyone play at all.
Focus on how to make wins, not on how bad that Pantheon on your team was.